1. Field of the Invention
The present invention relates to transconductance amplifiers. More particularly, the present invention relates to a nonlinear transconductance amplifier suitable to provide a nonlinear response to a differential signal. The nonlinear transconductance amplifier of the present invention is suitable for use as a nonlinear element in the feedback loop of a switching-type closed loop system, such as a DC--DC converter, for improving the response time to load transients, or a phase lock loop for improving the frequency capture rate. It may also be employed in a linear regulator.
2. The Prior Art
Traditional linear and more conventional switched mode power supply topologies are well known to those of ordinary skill in the art. In a typical linear power supply or voltage regulator circuit, a linear control element, such as a pass transistor, in series with an unregulated DC is used, with feedback, to maintain a constant output voltage. The output voltage is always lower in voltage than the unregulated input voltage, and some power is dissipated in the control element. Though the linear power supply has a fast response time, it is not very efficient in comparison to a switched mode converter when the input to output voltage ratio is large.
Typically, in a switching converter, a transistor operated as a saturated switch periodically applies the full unregulated voltage across an inductor for short intervals. The current in the inductor builds up during each pulse, storing 1/2 LI.sup.2 of energy in its magnetic field. The stored energy is transferred to a filter capacitor at the output that also smooths the output by carrying the output load between the charging pulses. In order to accommodate rapid and transient load changes, and to filter the switch frequency from the output, the output capacitor preferably has a large value with a very low equivalent series resistance (ESR). With feedback, the output of the converter is compared with the input to control the switching frequency or pulse width of the signal applied to the transistor operated as a switch. Since the control element is either off or saturated, the power dissipation in the regulator is minimized. Accordingly, switching regulators are very efficient, even when there is a large voltage drop from the input to the output.
In FIG. 1, a known DC--DC converter 10, referred to by those of ordinary skill in the art as a step-down or "buck" topology, is illustrated. In converter 10, a switch 12 is controlled by the output a comparator 14 having an inverting input connected to an oscillatory signal and a non-inverting input connected to a feedback signal to be described to form a pulse width modulator (PWM). A first terminal of switch 12 is connected to Vin, and a second terminal of switch 12 is connected a first terminal of inductor 16 and the anode of diode 18. The second terminal of inductor 16 is connected to a first plate of load capacitor 20, and also through voltage divider pair 22-1 and 22-2 to the inverting input of error amplifier 24. Vout is formed at the second terminal of inductor 16. A second plate of load capacitor 20 is connected to the cathode of diode 18 to complete a loop for current circulation, and also to a ground reference potential. The non-inverting input of error amplifier 24 is connected to a reference potential 26. The output of error amplifier 24 is connected to a first end of a resistor 28. A second end of resistor 28 is connected to a first plate of capacitor 30 and to the non-inverting input of comparator 14 to complete a feedback loop from Vout. A second plate of capacitor 30 is connected to ground.
In the converter 10, a high input voltage at Vin is converted to a lower input voltage at Vout. When the switch 12 is closed by the output of comparator 14, the voltage Vout-Vin is applied across the inductor 16, causing a linearly increasing current to flow through the inductor 16. When the switch 12 is opened, inductor current continues to flow in the same direction with the diode 18 conducting to complete the circuit. Since the voltage across the inductor 16 is now the difference between Vout and the nominal diode 18 voltage, the inductor current will decrease linearly. The load capacitor 20 operates to minimize current and voltage ripple at the output of the converter 10. It will be appreciated that as the size of the capacitor 20 increases, the amount of ripple decreases, however, the response time of the converter 10 to changes in the load also increases.
The feedback loop including the error amplifier 24 forms a control circuit to ensure that Vout remains at a selected value with a high degree of precision. In the feedback loop, Vout through the voltage divider 22-1 and 22-2 is compared to the reference voltage 26. The difference between the reference voltage 26 and Vout determines the width of the pulse driving the switch 12 from comparator 14 in a manner well understood by those of ordinary skill in the art.
The feedback control of the converter 10 has a finite response time to changing load conditions. The value of the output capacitor 20 is typically chosen to provide continuity in Vout while the feedback loop is responding to the changing load condition. The response times are typically slow in comparison to linear regulators, due to the size of the output capacitor 20. Further, the output capacitor is typically expensive due to its size, and also quite bulky.
The error amplifier 24 is typically a conventional operational amplifier which employs feedback filtering to provide either phase lag or phase advance responses. The feedback filtering often includes an arrangement of several capacitors. To aid stability and simplify compensation requirements, the error amplifier 24 is often low gain. To improve the response time of the feedback loop, it is known to implement the error amplifier 24 as a linear transconductance amplifier with a capacitive load, though for an increasing number of applications, the response time is inadequate.
For example, in conventional computer systems, the load conditions of a microprocessor may change very small amounts in a very short time due to very high microprocessor speeds or may change significantly in a very short time due to a state change in the microprocessor, such as power up. Further, various input and output devices of the computer system, such as hard drives or CD ROMS may change the load conditions of the microprocessor very rapidly.
Accordingly, it is an object of the present invention to provide a DC--DC converter with a fast response time to load changes.
It is yet another object of the present invention to reduce the size of the output capacitor in a step down DC--DC converter topology.
It is further object of the present invention to reduce the size of a step down DC--DC converter by reducing the size of the output capacitor.
It is yet a further object of the present invention to provide a nonlinear transconductance amplifier with a nonlinear response to a differential input
It another object of the present invention to simplify the phase compensation in an error amplifier of a DC--DC converter.